Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high-temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys, though without a protective coating components formed from superalloys typically cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. One such type of coating is referred to as an environmental coating, i.e., a coating that is resistant to oxidation and hot corrosion. Environmental coatings that have found wide use include diffusion aluminide coatings formed by diffusion processes, such as a pack cementation process.
Diffusion processes generally entail reacting the surface of a component with an aluminum-containing gas composition to form two distinct zones, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel or cobalt, depending on the substrate material. The MAl intermetallic is the result of deposited aluminum and an outward diffusion of iron, nickel or cobalt from the substrate. During high temperature exposure in air, the MAl intermetallic forms a protective aluminum oxide (alumina) scale that inhibits oxidation of the diffusion coating and the underlying substrate. The chemistry of the additive layer can be modified by the presence in the aluminum-containing composition of additional elements, such as chromium, silicon, platinum, rhodium, hafnium, yttrium and zirconium. Beneath the additive layer is a diffusion layer containing various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. The intermetallics within the additive layer are the products of all alloying elements of the substrate and diffusion coating.
Though significant advances have been made with environmental coating materials and processes for forming such coatings, there is the inevitable requirement to repair these coatings under certain circumstances. For example, removal may be necessitated by erosion or thermal degradation of the diffusion coating, refurbishment of the component on which the coating is formed, or an in-process repair of the diffusion coating or a thermal barrier coating (if present) adhered to the component by the diffusion coating. The current state-of-the-art repair method is to completely remove a diffusion aluminide coating by treatment with an acidic solution capable of interacting with and removing both the additive and diffusion layers. This process relies on lengthy exposures to stripping chemicals, often at elevated temperatures, that cause complete removal of the additive and diffusion layers, and can cause significant attack of the underlying metallic substrate, such as alloy depletion and intergranular or interdendritic attack. Substrate attack is most severe when a component being stripped has regions with different coating thicknesses or has uncoated surface regions, such as the dovetail of a turbine blade. A thicker coating requires longer exposure than does a thinner coating, with the result that the substrate beneath a thinner coating can be exposed to attack by the stripping solution for a significant length of time. For gas turbine blade and vane airfoils, removal of the diffusion layer and substrate attack can produce excessively thinned walls and drastically altered airflow characteristics.
From the above, it can be appreciated that improved methods for rapidly removing a diffusion aluminide coating are desired, particularly an improved method that does not significantly attack the substrate material underlying the coating.